Buggy code:
public class StringEscapeUtilsTest extends TestCase { private static final String FOO = "foo" ; public StringEscapeUtilsTest ( String name ) { } public static void main ( String [ ] args ) { } public static Test suite ( ) { } public void testConstructor ( ) { } public void testEscapeJava ( ) throws IOException { } public void testEscapeJavaWithSlash ( ) { } private void assertEscapeJava ( String escaped , String original ) throws IOException { } private void assertEscapeJava ( String message , String expected , String original ) throws IOException { } public void testUnescapeJava ( ) throws IOException { assertEquals ( null , StringEscapeUtils . unescapeJava ( null ) ) ; try { StringEscapeUtils . unescapeJava ( null , null ) ; fail ( ) ; } catch ( IOException ex ) { fail ( ) ; } catch ( IllegalArgumentException ex ) { } try { StringEscapeUtils . unescapeJava ( null , "" ) ; fail ( ) ; } catch ( IOException ex ) { fail ( ) ; } catch ( IllegalArgumentException ex ) { } try { StringEscapeUtils . unescapeJava ( "\\u02-3" ) ; fail ( ) ; } catch ( RuntimeException ex ) { } assertUnescapeJava ( "" , "" ) ; assertUnescapeJava ( "test" , "test" ) ; assertUnescapeJava ( "\ntest\b" , "\\ntest\\b" ) ; assertUnescapeJava ( "ሴ25foo\ntest\b" , "\\u123425foo\\ntest\\b" ) ; assertUnescapeJava ( "\'\foo\teste\r" , "\\\'\\foo\\teste\\r" ) ; <START_BUG> assertUnescapeJava ( "\\" , "\\" ) ; <END_BUG> assertUnescapeJava ( "lowercase<seq2seq4repair_space>unicode" , "ꯍx" , "\\uabcdx" ) ; assertUnescapeJava ( "uppercase<seq2seq4repair_space>unicode" , "ꯍx" , "\\uABCDx" ) ; assertUnescapeJava ( "unicode<seq2seq4repair_space>as<seq2seq4repair_space>final<seq2seq4repair_space>character" , "ꯍ" , "\\uabcd" ) ; } private void assertUnescapeJava ( String unescaped , String original ) throws IOException { } private void assertUnescapeJava ( String message , String unescaped , String original ) throws IOException { } public void testEscapeJavaScript ( ) { } String [ ] [ ] htmlEscapes = new String [ ] [ ] { new String [ ] { "no<seq2seq4repair_space>escaping" , "plain<seq2seq4repair_space>text" , "plain<seq2seq4repair_space>text" } , new String [ ] { "no<seq2seq4repair_space>escaping" , "plain<seq2seq4repair_space>text" , "plain<seq2seq4repair_space>text" } , new String [ ] { "empty<seq2seq4repair_space>string" , "" , "" } , new String [ ] { "null" , null , null } , new String [ ] { "ampersand" , "bread<seq2seq4repair_space>&amp;<seq2seq4repair_space>butter" , "bread<seq2seq4repair_space>&<seq2seq4repair_space>butter" } , new String [ ] { "quotes" , "&quot;bread&quot;<seq2seq4repair_space>&amp;<seq2seq4repair_space>butter" , "\"bread\"<seq2seq4repair_space>&<seq2seq4repair_space>butter" } , new String [ ] { "final<seq2seq4repair_space>character<seq2seq4repair_space>only" , "greater<seq2seq4repair_space>than<seq2seq4repair_space>&gt;" , "greater<seq2seq4repair_space>than<seq2seq4repair_space>>" } , new String [ ] { "first<seq2seq4repair_space>character<seq2seq4repair_space>only" , "&lt;<seq2seq4repair_space>less<seq2seq4repair_space>than" , "<<seq2seq4repair_space>less<seq2seq4repair_space>than" } , new String [ ] { "apostrophe" , "Huntington's<seq2seq4repair_space>chorea" , "Huntington's<seq2seq4repair_space>chorea" } , new String [ ] { "languages" , "English,Fran&ccedil;ais,&#26085;&#26412;&#35486;<seq2seq4repair_space>(nihongo)" , "English,Français,日本語<seq2seq4repair_space>(nihongo)" } , new String [ ] { "8-bit<seq2seq4repair_space>ascii<seq2seq4repair_space>doesn't<seq2seq4repair_space>number-escape" , "~" , "~" } , new String [ ] { "8-bit<seq2seq4repair_space>ascii<seq2seq4repair_space>does<seq2seq4repair_space>number-escape" , "&#128;&#159;" , "" } } ; public void testEscapeHtml ( ) { } public void testUnescapeHtml ( ) { } public void testUnescapeHexCharsHtml ( ) { } public void testUnescapeUnknownEntity ( ) throws Exception { } public void testEscapeHtmlVersions ( ) throws Exception { } public void testEscapeXml ( ) throws Exception { } public void testStandaloneAmphersand ( ) { } public void testLang313 ( ) { } public void testEscapeCsvString ( ) throws Exception { } public void testEscapeCsvWriter ( ) throws Exception { } private void checkCsvEscapeWriter ( String expected , String value ) { } public void testUnescapeCsvString ( ) throws Exception { } public void testUnescapeCsvWriter ( ) throws Exception { } private void checkCsvUnescapeWriter ( String expected , String value ) { } public void testEscapeHtmlHighUnicode ( ) throws UnsupportedEncodingException { } }

Fix:
assertUnescapeJava ( "" , "\\" ) ;